1. Field of the Invention
The present disclosure relates to imaging optical systems and image pickup apparatuses including the imaging optical systems, and is suitable for an imaging optical system included in, for example, a digital still camera, a digital video camera, a TV camera, a monitoring cameras, or a silver-halide film camera.
2. Description of the Related Art
Imaging optical systems included in single-lens reflex digital still cameras and single-lens reflex silver-halide film cameras are required to have a large imaging angle of view of about 40° to 60°. Such an imaging optical system is also required to have a predetermined back focus so that an optical member, such as a filter, may be arranged on the image side of the imaging optical system. Furthermore, the imaging optical system is also required to achieve high optical performance, have a large aperture ratio, and be capable of performing quick focusing. When the aperture ratio is increased, the shutter speed can be increased so that camera shake during, for example, indoor shooting can be easily suppressed.
In an imaging optical system, focusing is generally performed by moving the entire imaging optical system or a part of the imaging optical system. For example, an inner focus system is known which performs focusing by moving an intermediate lens unit included in the imaging optical system. With the inner focus system, the amount of movement of a focusing lens unit is smaller than that in the case where the entire imaging optical system is moved, and the size and weight of the focusing lens unit can be easily reduced.
As a result, the focusing lens unit can be moved at a high speed during focusing, and the drive torque applied during focusing can be reduced. Therefore, in the case where, for example, the system is included in an imaging apparatus including an automatic focus detection device, the size of a drive motor can be easily reduced.
Inner focus imaging optical systems having a relatively large imaging angle of view and a long back focus have been known. U.S. Pat. No. 4,852,984 and Japanese Patent Laid-Open No. 1-185507 disclose imaging optical systems which include, in order from an object side to an image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power, and in which focusing is performed by moving the second lens unit.
In inner focus systems, focusing can be performed by moving a relatively small and light lens unit, so that high-speed focusing can be facilitated. In addition, a minimum imaging distance, at which imaging can be performed, is small.
However, compared to the case in which the entire optical system is moved during focusing, in the inner focus system, large variations in aberrations generally occur when the focusing lens unit is moved, and it is difficult to appropriately correct the aberrations over the entire object distance range. For example, in an inner focus system in which focusing is performed by moving an intermediate lens unit arranged near an aperture stop of the imaging optical system, large variations in spherical aberration and coma aberration occur during focusing. The tendency of variations in aberrations becomes more significant when the aperture ratio of the imaging optical system is increased.
When the aperture ratio of an inner focus imaging optical system having a large imaging angle of view of about 40° to 60° is increased, the thickness of each lens is increased. As a result, the back focus tends to be reduced to ensure enough space for the movement of the focusing lens unit.
Therefore, to increase the angle of view and the aperture ratio of an inner focus imaging optical system, it is necessary to appropriately set the refractive powers and lens configurations of lens units of the imaging optical system including the focusing lens unit. Unless the lens units are appropriately configured, large variations in aberrations occur during focusing and it becomes difficult to achieve high optical performance over the entire object distance. In particular, it becomes difficult to increase the back focus while increasing the angle of view and the aperture ratio.